For the purpose of bearing very fast-rotating working means, for example spinning rotors of open-end spinning machines, there are special devices which by means of controlled magnetic forces maintain the position of the rotating working means. Such devices are generally known as active magnetic bearings.
In order to ensure levitation of the rotating working means, which is the function of the bearing, by means of the active magnetic bearing, it is essential to know the instantaneous position of the rotating working means in the magnetic field of the active magnetic bearing, as well as possible changes of this position over time in a three-dimensional rectangular coordinate system with the axes x, y, z, which is a fundamental prerequisite for the active stabilization of the rotation axis of the rotating working means in space. Also, the rotating working means must be maintained in the required position by a regulation (controlling) device on the basis of continuously registered and evaluated data in order to avoid an accident of the rotating working means under the influence of variations of the rotating working means leading to all the negative consequences caused by the high speed of the rotation of the rotating working means, e.g. an accident due to the contact of the rotating working means with the other parts of the active magnetic bearing.
To determine the position of the rotating working means in a magnetic field, it is usual to use sensors based on employing the principle of vortex currents by means of a measuring electrical coil or a pair of coils with a mutual inductive coupling. The change of the position of the rotor in the magnetic field of the active magnetic bearing evokes a change in AC voltage on the measuring electrical coil, which is actuated by AC voltage either through direct coupling or through inductive coupling from the other coil. The output signal of the coil in the form of the AC voltage on the measuring coil is subsequently processed by a detector. The amount (amplitude) of this AC voltage induced then depends on the position of the spinning rotor in the magnetic bearing, whereby the value of the amount of the voltage induced is obtained with the aid of the above-mentioned detector, which in actual fact carries out the regulation of the AC voltage induced, and so the amount of the regulated voltage induced corresponds to the position of the spinning rotor with respect to the position sensor. Known systems of induced voltage usually work with frequencies in the order of tens to hundreds of kilohertz, possibly in the order of single megahertz, which requires using sensing coils with a plurality of threads, or, as the case may be, it requires using flat coils having relatively large dimensions, usually disposed in a plane perpendicular to the rotation axis of the working means. The production of such coils is generally technologically demanding with regard to serial production requirements, such as small tolerances, parameter stability and low cost. Moreover, when using larger coils, a problem arises during detecting the position of a rotating working means whose diameter is small in comparison with these coils, as is, for example, the diameter of the driving shaft.
The simplest known detectors applicable in this field are plain or various types of compensated diode rectifiers, whose disadvantage, however, is the fact that they are not able to detect small induced voltages in the order of milivolts with sufficient accuracy and stability. Another drawback of these detectors is their temperature dependability. So as to achieve greater accuracy or temperature stability, it is often necessary to set these detectors individually. In some cases also an amplifier, often a selective one, is positioned between a sensing coil and a detector. See, for example, CZ 302 646.
Generally speaking, a method of detecting small signals which uses various types of synchronnic or generally controlled rectifiers with semiconductive switches is considered to be very accurate. However, the disadvantage is a certain limitation of the working frequency in the order of single MHz and mostly also a greater complexity and therefore the price of detectors which are based on this principle. Nevertheless, in active magnetic bearings in which high speed is achieved—as high as more than 100.000 min−1, even in the case of these detectors a problem emerges with detection and evaluation of very low AC voltages coming from the sensors of the position of the spinning rotor working on the above-mentioned principle of vortex currents, which is caused especially by the effect of the electromagnetic disturbance arising during the operation of the active magnetic bearing, particularly its actuator coils. Also, the working frequency of the actuator coils of the active magnetic bearing tends to be low, often nearing the frequency at which the detectors work, which means that it is usually difficult to eliminate this disturbance.
Generally, a method of using detectors of alternating signals is known, in which the detectors are based on the principle of non-linearity of a transfer characteristic of a quadrupole, preferably the so-called field-effect transistor. However, the output signals from such detectors are not directly proportional to changes of AC voltage on the measuring electrical coils of the position sensors and therefore it is usually necessary to correct them by a DC correction signal.
CZ patent No 302 646 discloses a method of stabilization of a levitating rotating element (LRE) by means of high frequency inductive position sensors connected to evaluation circuits (detectors) of output signals of sensors. In actual fact, the whole system is made up of a flat high-speed electrical motor with a magnetic bearing whose rotor is composed of a levitating and rotating element, i.e. of the spinning rotor of an open-end spinning machine. Inductive position sensors LRE are arranged in close proximity to the outer circuit LRE and are disposed along the circumference of LRE in three pairs, whereby all these three pairs of position sensors are disposed in one common plane perpendicular to the rotation axis LRE. High frequency inductive position sensors work at frequencies in hundreds of kHz. LRE is made of ferromagnetic material, whereby each of the high frequency inductive position sensors is in essence made up of a flat transformer with flat windings arranged on opposite sides of the printed circuit board and lying in a plane perpendicular to the rotation axis of the spinning rotor. Primary windings (actuator windings) of both sensors of each pair of sensors are connected to the branch of the power output of a high frequency (actuator) generator of a specific signal, secondary windings of one sensor of each pair of sensors is connected with the inlet of a narrow-band elimination filter and secondary winding of the other of each pair of sensors is connected to the inlet of the other narrow-band elimination filter. The output signal of each pair of sensors is first processed (amplified) on AC level and only afterwards it is supplied to a high frequency rectifier, to the outlet of which is connected the inlet of a smoothing filter, which is by its outlet connected to the inlet of a digital signal processor. Although this arrangement to a certain extent reduces the negative influence of the disturbing background created by the principle itself of the function of the magnetic bearing due to using narrow-band filters in the path of AC signal, its disadvantages are complexity and insufficient thermal stability. Moreover, this arrangement requires the individual setting of the central position of the rotation axis of LRE (rotor) by means of setting elements, which makes the device even more demanding and it is difficult to find practical applications for this solution. In addition, this solution does not enable to detect a possible inclination of the rotation axis of LRE in relation to the optimal rotation axis of LRE, i.e. tilting of the rotor, whereby this possible inclination can lead to to the contact of the fast-rotating LRE with the other parts of the active magnetic bearing, which may result in an accident of the whole system, including destruction of the device and threatening the health of the operating staff. Due to a relatively low working frequency of the position sensors, the system is also sensitive to the material of which the spinning rotor is made in the area adjacent to the sensor, since the depth of penetration of the electromagnetic field into the material of the rotor is considerably great. The surface phenomenon, the so-called skin effect, is applied here only to a lesser extent.
The aim of the invention is to eliminate or at least reduce the drawbacks of the background art, especially to improve the parameters of detecting the position of the rotating working means in an active magnetic bearing.